Dakin oxidation template

Hydroperoxide adds to the C=O of salicylaldehyde; the electron-rich aryl group migrates to oxygen as hydroxide leaves (as in the Baeyer–Villiger), and the aryl formate is hydrolysed to catechol.

Mechanism step by step

Alkaline H₂O₂ turns an o- or p-hydroxybenzaldehyde into a benzenediol and formate

1. Hydroperoxide adds – HOO⁻ (from H₂O₂ and NaOH) adds to the C=O: a tetrahedral intermediate
2. Aryl migration (slow) – The C=O reforms, the aryl group moves to O and hydroxide leaves: an aryl formate
3. Hydroxide adds to the ester – Ester hydrolysis begins: a second tetrahedral intermediate
4. Phenoxide leaves – Collapse releases the phenoxide and formic acid (formate in base); acid work-up gives catechol

Key points

• Needs an electron-donating OH (or NH₂) ortho or para to the CHO: p-hydroxybenzaldehyde gives hydroquinone, salicylaldehyde gives catechol.
• The migration step is the Baeyer–Villiger step: the electron-rich aryl group (helped by the phenoxide in base) migrates in preference to H.
• With no donor, or a donor meta to the CHO, H migrates instead and the product is the benzoic acid.
• Mild and cheap (H₂O₂, NaOH, or sodium percarbonate): a route to phenols from aldehydes.

Drawn with 108 atoms, 14 arrows, 8 shapes – every one editable in MolDraw.

Sources

• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020 Textbook